1. Field of the Invention
The present invention relates to a guide wire for guiding the insertion of a catheter into a blood vessel.
2. Description of the Related Art
A guide wire for a catheter is required to have several characteristics, including good flexibility as well as resilience against and recovery from deformation. Also, the guide wire has to be smoothly inserted into and along blood vessels within the human body, which branch and meander in the body, without damaging the blood vessel.
Conventionally, wires made of plastic, carbon steel, stainless steel or the like are used as the guide wires for catheters. More recently, guide wires having their core wire made of a superelastic metal, such as a nickel-titanium alloy have been proposed. See, for example, Japanese Laid open Patent Publication Nos. 2-24548, 2-24549 and 2-24550.
Superelasticity refers to a characteristic that, even if a material is strained beyond its yield point such that it appears to have undergone plastic deformation, when the external stress is relieved, the material returns to the original shape. As shown by the curve E3 FIG. 10, the load-elongation diagram for a superelastic alloy includes a horizontal portion P at which the resistance load of the alloy does not increase even as the alloy elongates. The load-elongation also has a proportional portion Q where the elongation and the load change proportionally. The superelastic alloy is therefore more flexible and more elongatable than other metals, such as stainless steel.
In the environment of a core wire of a catheter guide wire, if a sufficient load is applied to the core wire to bring the alloy into the horizontal portion P, the core wire merely elongates. This means that the guide wire becomes too flexible. Thus, it is difficult for the user to sense the movements and response of the guide wire during use and buckling occurs. This impedes the delicate control necessary to insert the guide wire into the blood vessel.
To overcome this shortcoming, some countermeasure may be taken to increase the diameter of the guide wire to give it a certain rigidity. Increasing of the diameter of the guide wire however restricts (or adversely influences) the operability of the guide wire. It also raises the cost of the guide wire, which is undesirable.
Further, increasing the diameter of the guide wire results in a steeper change of the previously described loadelongation curve. Thus, the characteristics of such a guide wire are not very different from the characteristics of the conventional stainless-steel wire. The use of such guide wires tend to be painful for the patients. For example they tend to be painful during removal because the shape of the guide wire tends to change some and the bent portions of the guide wire are difficult to pull back through the tortuous path of the blood vessels.